Try looking at the fast path module.[0] Your router might have some kind of Qualcomm NAT accelerator chip[1] that isn't natively supported/distributed by OpenWRT.[2] You'll have to compile your own build of OpenWRT, so it might be a bit difficult to get going. I haven't needed to do it, because my WAN connection is only 50 Mbps.

[0] https://github.com/gwlim/openwrt-sfe-flowoffload

[1] https://forum.openwrt.org/t/qualcomm-fast-path-for-lede/4582

[2] like mine https://openwrt.org/toh/wd/n750